The plica semilunaris is a narrow, highly vascular, crescent-shaped fold of the conjunctiva located lateral to and partly under the caruncle (see Fig 1-24). Its lateral border is free and separated from the bulbar conjunctiva, which it resembles histologically. The epithelium of the plica is rich in goblet cells. The plica’s stroma contains fat and some nonstriated muscle. The plica is a vestigial structure analogous to the nictitating membrane, or third eyelid, of dogs and other animals.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.